1. Field of the Invention
Embodiments of the present invention generally relate to temperature control of a reactor using probability distribution of temperature measurements.
2. Description of the Related Art
Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis.
Maleic anhydride is conventionally manufactured by passing a gas comprising a hydrocarbon having at least four carbon atoms in a straight chain and oxygen through a catalyst bed, typically a fixed catalyst bed tubular plug flow reactor, containing a catalyst including mixed oxides of vanadium and phosphorus. The catalyst employed may further comprise promoters, activators or modifiers such as iron, lithium, zinc, molybdenum, chromium, uranium, tungsten, and other metals, boron and/or silicon. The product gas exiting the reactor typically contains maleic anhydride together with oxidation by-products such as carbon monoxide, carbon dioxide, water vapor, acrylic and acetic acids and other by-products, along with inert gases present in air when air is used as the source of molecular oxygen.
Because the reaction is highly exothermic, the reactor must be cooled during operation. Typically, a shell and tube heat exchanger is used as a reactor with the catalyst packed in the tubes through which the hydrocarbon and oxygen gases are passed. A cooling fluid, often a molten salt, flows over and cools the outside of the tubes. Because the length of the tubes is generally much greater than the diameter of the tubes, the reaction system approaches plug flow.
While the cooling capacity is substantially uniform throughout the reactor, the rate of reaction varies widely with the concentration of the hydrocarbon reactant and the temperature of the reaction zone. Because the reactant gases are generally at a relatively low temperature when they are introduced into the catalyst bed, the reaction rate is low in the region immediately adjacent the inlet of the reactor. Once the reaction begins, however, it proceeds rapidly with the rate of reaction further increasing as the reaction zone temperature increases from the heat released by the reaction. The reaction zone temperature continues to increase with distance along the length of the reactor tube until the depletion of the hydrocarbon causes the rate of reaction to decrease thereby decreasing the temperature of the reaction zone through transfer of heat to the cooling fluid, and allowing the remaining portion of the reactor tube to operate at a lower temperature differential. In practice, commercial reactors are configured so that a number of tubes, typically 50-100+, are equipped with a longitudinal thermocouple in the center of the tube, inserted to a tube depth (distance from the top or bottom tubesheet) where maximum temperatures are expected. Of these multiple measurement locations, the location with the highest temperature is generally referred to as the “hot spot”.
If the temperature distribution in the reactor increases, reactor performance, catalyst activity, and the integrity of the reactor vessel may deteriorate. Generally, the selectivity of the catalyst varies inversely with the reaction temperature while the rate of reaction varies directly with the reaction temperature. Higher reaction zone temperatures result in lower catalyst selectivity and favor the complete oxidation of the hydrocarbon feedstock to carbon dioxide and water instead of maleic anhydride. As the temperature distribution in the reactor increases, the amount of the hydrocarbon feedstock consumed by the reaction increases but the decreased selectivity of the catalyst can result in a decreased yield of maleic anhydride. In addition, exposure of the catalyst bed to excessive temperatures may degrade the catalyst activity and cause and excessive rate of corrosion of the reactor tubes. Such degradation of the catalyst activity generally reduces the productivity of the operation and may also reduce the selectivity of the catalyst at a given temperature. The higher heat of reaction released by the conversion of the hydrocarbon feedstock to carbon dioxide and water further compounds this problem. An excessive rate of corrosion of the reactor tubes will lead to premature failure of individual tubes or of the entire reactor.
Typically, the catalyst bed temperature is continuously monitored at 50-100+ tubes via a single thermocouple at each location. The bulk of the catalyst bed is maintained below an upper temperature limit by reducing the feed rate of the limiting reactant (i.e., air or butane) if the “hot spot” is above the specified upper temperature limit.